1. Field of the Invention
The present invention relates to an objective lens and an optical pickup apparatus having the objective lens and arranged to read and write an information signal to and from an optical recording medium, such as an optical disk, a magneto-optical disk or an optical card.
2. Description of Related Art:
Hitherto, optical recording mediums, such as optical disks, magneto-optical disks and optical cards, have been widely used to store data of dynamic image information, voice information and data for computers because the optical recording mediums can easily be manufactured and the cost can be reduced. In recent years there is more need for raising the density of information signals, which can be recorded, and enlarging the capacity owning to rapid progress of the information society.
To raise the density of information signals, which can be recorded on an optical recording medium of the foregoing type, shortening of the wavelength of the laser beam for reading the information signal and raising of the NA {(that is, use of an objective lens having a high NA (the number of apertures)} of the objective lens for converging the laser beams onto the optical recording medium are effective means. The reason for this lies in that the minimum size of a beam spot, which is formed by converging the laser beams, cannot be reduced to .lambda./NA (.lambda.: the wavelength of the light beam) or smaller.
To shorten the wavelength of the laser beam, a blue laser diode, a blue SHG laser and a green SHG laser have been developed. On the other hand, raising of the NA of the objective lens has been attempted by making the NA of the objective lens of a so-called "digital video disk (DVD)" (a digital optical disk adapted to a video signal) having a recording density higher than that of a so-called "compact disk (CD)" (digital optical disk adapted to an audio signal or computer data) to be 0.6 in comparison to the NA of the "compact disk (CD)" which is 0.45. The objective lens of the optical disk is formed into an aspheric single lens (a monocyte aspheric lens) made of a synthetic resin material or a glass material.
To eliminate influence of a coma aberration occurring because of an inclination of the "digital video disk (DVD)", the substrate of the "digital video disk (DVD)" has a thickness of 0.6 mm which is half of that of the substrate of the "compact disk" and that of the magneto-optical disk.
In order to further raise the density of information signals, which can be recorded, as compared with the density realized by the "digital video disk (DVD)", an objective lens having an NA higher than 0.6 is required.
However, to manufacture an objective lens having an NA not lower than 0.7, various requirements must be satisfied.
An objective lens having a high NA suffers from chromatic aberration which is generated attributable to change in the wavelength of a semiconductor laser (vertical-mode hop which takes place when the environmental temperature is changed). Since the conventional monocyte objective lens has an NA which is not higher than 0.6 with which chromatic aberration is not generated considerably, the lens of the foregoing type can be made of optical glass, the Abbe's number of which is 50 or less and which therefore has relatively high diffusion and high refractivity. Since the cost of the optical glass, having high diffusion and frequency, can be reduced, the foregoing optical glass can satisfactorily be mass-produced. Therefore, the foregoing material has been widely used.
However, high NA objective lenses of a type having an NA of 0.7 or higher encounter great chromatic aberration if the foregoing objective lens is made of high diffusion optical glass. In this case excessive defocus takes place on the surface of an optical disk on which a signal is recorded. Therefore, chromatic aberration must be prevented by using low-diffusion optical glass.
Since the major part of the low-diffusion optical glass has a low refractivity, the curvature of the surface is sharpened excessively if an objective lens having a short focal distance and a high NA is manufactured. In this case, a mold for manufacturing the lens cannot easily be machined. The present level of the technique for machining the aspheric surface cannot accurately manufacture a mold by using a diamond bite if the angle .theta. made between the contact surface of the aspheric surface and a plane perpendicular to the optical axis is larger than 50 degrees (according to a report satisfactory lenses have been obtained when the angle .theta. is about 55 degrees or smaller).
However, the objective lens having a short focal distance and a high NA is usually designed to have the above-mentioned angle .theta. which exceeds 55 degrees. In this case, permissible decentering for the distance between two sides of the lens when the mold or the lens is manufactured is reduced excessively. Thus, the manufacturing efficiency deteriorates excessively.
It might therefore be considered feasible to employ a doublet lens structure to distribute the curvature to the four surfaces. However, even the doublet lens attempted to maintain a satisfactorily long working distance involves excessively sharp curvature of the surface. Moreover, permissible decentering between the surfaces of the lens and the permissible angle of field to be reduced when the lens is manufactured. Thus, the manufacturing efficiency deteriorates excessively. To reduce the aperture of an objective lens, that is, to reduce the diameter of an objective lens is an important fact because the reduction enables the size of the optical pickup apparatus to be reduced and thus an economic advantage can be realized. To maintain a sufficiently long working distance is an important factor to prevent contact between the objective lens and the optical disk which is rotated at high speed.
Therefore, the doublet objective lens must comprise a lens having a gentle curvature of the surface without deterioration in the manufacturing efficiency.
Although the curvature of the objective lens can be made to be gentle and thus the manufacturing efficiency of the objective lens can be improved if the aperture of the objective lens is enlarged, the weight of the portion including the objective lens is enlarged. In this case the size of the optical pickup apparatus cannot be reduced. Moreover, the performance of the actuator (a mechanism for driving the objective lens) for moving the objective lens to follow the optical disk must be improved. In this case, the size and cost of the optical pickup apparatus cannot be reduced.
If the objective lens having a high NA is employed, there arises another problem in that the RF signal deteriorates even with a small amount of disk skew and the signal cannot easily be reproduced from the optical disk because the coma-aberration, which is generated attributable to the skew of the optical disk, is enlarged in proportion to the cube of the NA.